This invention relates to a method for the determination of biomass in a medium, to a fermentation process in which the method is used as a control feature, and to apparatus for carrying out both the method and the process.
There is increasing interest in biotechnology, both in the traditional fermentation industry and in the exploitation of living cells in new processes to produce commercially useful products such as antibiotics, vitamins, amino acids and a variety of biologically active proteins.
The productivity of fermentation processes is dependent to a considerable extend upon culture conditions. It is therefore desirable, and has now become conventional, to control those variable such as pH and dissolved oxygen tension for which sensors are available.
One of the most important variables in a fermentation process is the reactor biomass concentration, i.e. the concentration of microbial or other cells in the fermenter, since the productivity under a given set of process conditions is directly proportional to this. However, to date no accurate means has been developed for measuring the biomass content of a culture in real time, i.e. for measuring the present biomass content rather than the biomass content some time in the past.
The lack of suitable means to measure this important process variable has been commented upon by several writers in recent years, see for example: Pirt, "Principles of Microbe and Cell Cultivation", Blackwell, 1975, p 16; Carleysmith and Fox, "Fermenter Instrumentation and Control," Adv. Biotechnol. Processes 3, 1-51, 1984; and Harris and Kell, "The estimation of Microbial Biomass," Biosensors J.1, 17-84 (1985).
This last reference notes (1) that the most appropriate measure suitable for estimating biomass in real time is the biovolume, i.e. the volume fraction of a culture enclosed by the cytoplasmic membrances of the microbial or other cells within it, (2) that the only means by which biomass might be estimated in real time will be by physical as opposed to chemical measurements, and (3) that all presently available physical methods such as light scattering) for estimating biomass, are essentially unusable under the difficult conditions existing in a fermenter.
It is an object of the present invention to provide a measurement of the microbial or other biomass in fermenters in real time and preferably in situ.